1. Field of the Invention
The present invention relates to a semiconductor device, and a method for making the same. More particularly, the present invention relates to a semiconductor device containing such a semiconductor chip as a ferroelectric random access memory which will become unstable in operation under a high temperature, and a method for making such a semiconductor device.
2. Background Art
Much effort is being made in recent years in developing a nonvolatile memory utilizing the spontaneous polarization behavior of a ferroelectrics having a high dielectric constant. This type of memory is usually called the ferroelectric random access memory (hereinafter called FRAM). The FRAM has a construction in which a planer type ferroelectric capacitor is formed on a layer of a common CMOS transistor. The FRAM enables overwriting of stored information at an extremely high speed and at a low voltage by reversing the direction of polarization.
However, when heated, the ferroelectrics used in the FRAM loses the ferroelectricity above a certain temperature (Curie temperature), and becomes paraelectric in which state the spontaneous polarization will not occur. A common Curie temperature of the ferroelectrics used in the FRAM is 170-180 degrees Celsius. If the FRAM containing such a ferroelectrics is heated to this Curie temperature or above, the memory operation becomes unstable, or the memory will no longer operate. In other words, the FRAM is susceptible to heat.
On the other hand, not only the FRAM but also the semiconductor chip in general includes pieces of metal wire for example, for establishing electrical connection between electrodes of the semiconductor chip and external components such as inner leads of the lead frame and a circuit pattern formed on a substrate. This type of electrical connection using the metal wire is commonly performed by a method such as thermocompression bonding or ultrasonic bonding.
The thermocompression bonding is performed as follows. Specifically, a component element to which the bonding is to be made is heated to a relatively high temperature (about 400 degrees Celsius). Then, the metal wire is pressed onto a wirebonding region. However, this method is not suitable for wirebonding to a semiconductor chip which is susceptible to heat, because the semiconductor chip has to be heated up to 400 degrees Celsius. On the other hand,the ultrasonic bonding is performed without heating the component element to be bonded. Specifically, the metal wire is first pressed onto the bonding region, and then ultrasonic wave is applied. However, the ultrasonic bonding method is disadvantageous in that an intense ultrasonic wave can break the wire.
In order to compensate for the above shortcomings in the thermocompression bonding and the ultrasonic bonding, thermosonic bonding is often employed. Specifically, the component element to be bonded is heated only to a relatively low temperature (about 200 degrees Celsius). Then, the metal wire is pressed onto the bonding region, and ultrasonic wave of moderate intensity is applied.
Still however, the thermosonic bonding is only suitable for wirebonding a common type of semiconductors because the semiconductor chip must be heated to about 200 degrees Celsius. The thermosonic bonding is still not suitable for wirebonding a semiconductor chip such as the FRAM which is extremely susceptible to heat, becoming very unstable in operation if heated to 170-180 degrees Celsius.
There is another problem. Specifically, a surface of the semiconductor chip is commonly formed with bonding pads for wirebonding. These pads are formed by aluminum for example. However, aluminum is easily oxidized to form a coat of oxide, which weakens the bond between the bonding pad and the wire. This problem becomes more significant at a higher bonding temperature, and in order to remove the coat of oxide, the ultrasonic wave of a greater intensity must be applied at a risk of breaking the metal wire.
It is therefore an object of the present invention to provide a semiconductor device in which the electrode of the semiconductor chip and the component element to be connected with the electrode are connected appropriately with each other without damaging a property of the semiconductor chip.
Another object of the present invention is to provide a semiconductor device including a ferroelectric memory chip in which the electrode of this memory chip and the component element to be connected with the electrode are connected appropriately with each other without heating the ferroelectric memory chip beyond a predetermined temperature.
According to a first aspect of the present invention, there is provided a semiconductor device having a following arrangement.
Specifically, the semiconductor device comprises a semiconductor chip having a main surface formed with an electrode pad, a package containing the semiconductor chip, a component element electrically connected with the electrode pad within the package, a gold bump formed on the electrode pad, and a gold wire having an end bonded to the gold bump and the other end bonded to the component.
According to the above arrangement, the electrode pad conventionally formed by aluminum is covered by the gold bump. This eliminates need for such an operation at the time of wirebonding as removing a coat of oxide formed on the electrode pad. Thus, there is no need for applying a large amount of energy at the time of wirebonding such as applying intense ultrasonic wave for removing the coat of oxide, or heating the semiconductor chip to a high temperature. Further, at the time of wirebonding, the gold bump absorbs part of pressure applied by the capillary. Thus, the semiconductor chips are better protected from damage caused by the wirebonding operation. Still further, the wire made of gold is bonded to the electrode pad made of gold. Since this bond is made between the same kind of metal, only a smaller amount of energy is necessary to achieve the bonding. Still further, the bond is not susceptible to oxidization, and therefore can keep a good quality of connection.
According to a preferred embodiment, the component element is another semiconductor chip. This another semiconductor chip is formed with an electrode pad formed with a gold bump thereon. The other end of the wire is bonded to the gold bump of said another semiconductor chip.
According to another preferred embodiment, the component element is a substrate. The substrate is provided with a terminal portion, and the other end of the wire is bonded to the terminal portion.
According to the preferred embodiment, the semiconductor chip is a ferroelectric memory chip. The ferroelectric memory chip is a nonvolatile memory chip utilizing the spontaneous polarization behavior of a ferroelectrics having a high dielectric constant. This type of memory enables overwriting of stored information at an extremely high speed and at a low voltage, by reversing the direction of polarization. The ferroelectrics used in the ferroelectric memory chip is susceptible to heat (becoming unable to polarize spontaneously at a temperature of 170-180 degrees Celsius). For this reason, the ferroelectric memory chip becomes unstable in operation once heated above a specific temperature. According to the present invention, in order to achieve the wirebonding between the electrode pad and the gold wire, a smaller amount of energy may be applied as described above. For example, the heating temperature may be about 100 degrees Celsius, and the ultrasonic wave may be less intense. Thus, it is possible to establish an appropriate electric connection between the ferroelectric memory chip and a component element to be connected to the ferroelectric memory chip, without the risk of damaging the operational stability of the ferroelectric memory chip.
According to a second aspect of the present invention, there is provided a semiconductor device having a following arrangement.
Specifically, the semiconductor device comprises a first semiconductor chip having a main surface formed with an electrode pad, a second semiconductor chip having a main surface faced to the main surface of the first semiconductor chip and formed with an electrode pad opposed to the electrode pad of the first semiconductor chip, and a package containing the first and the second semiconductor chips. Each of the electrode pads of the first and the second semiconductor chips is provided with a gold bump formed thereon. The first semiconductor chip and the second semiconductor chip are electrically connected with each other via a gold connecting member placed between the pair of the opposed gold bumps.
According to the above arrangement, the same advantages are obtained from the formation of the gold bump on the electrode pad of each semiconductor chip, as well as the other advantages as described above according to the first aspect of the present invention.
According to a preferred embodiment, the gold connecting member is a gold bump formed in the first semiconductor chip, or an easily deformable gold stud bump formed on the gold bump in the second semiconductor chip. When the electrical connection between the electrode pads of the two semiconductor chips is established via the gold stud, the electrode pad formed with the gold bump is pressed to the gold stud bump. During this operation, the stud bump which is easily deformable, protects the two semiconductor chips from damage caused by excessive pressure applied to the main surfaces of respective semiconductor chips faced to each other. It should be noted that the stud bump may be such as having a tip portion made easily deformable, or having another predetermined portion made easily deformable.
According to the preferred embodiment, one or both of the first semiconductor chip and the second semiconductor chip is a ferroelectric memory chip. As described earlier, the ferroelectric memory is susceptible to heat. According to the present invention, electrical connection between the first semiconductor chip and the second semiconductor chip can be established without applying a large amount of heat or an intense level of ultrasonic wave because of the easily deformable stud bump.
According to the preferred embodiment, the main surface of the first semiconductor chip and the main surface of the second semiconductor chip are bonded to each other by a resin adhesive. With this arrangement, the first semiconductor chip and the second semiconductor chip are mechanically connected by the resin adhesive while allowing the pair of electrodes each formed with the gold bump to be electrically connected to each other. Thus, a high quality of electric connection can be kept as mentioned earlier, and at the same time, the main surface of each semiconductor can be well protected.
According to the preferred embodiment, the resin adhesive is an epoxy resin or a phenol resin.
According to a third aspect of the present invention, a method for manufacturing a semiconductor device comprising the above described steps as part thereof.
Specifically, the semiconductor device manufactured by this method comprises a first semiconductor chip having a main surface formed with an electrode pad, a second semiconductor chip having a main surface faced to the first semiconductor chip and formed with an electrode pad opposed to the electrode pad of the first semiconductor chip, and a package containing the first and the second semiconductor chips. The first and the second semiconductor chips are electrically connected with each other within the package. The method comprises steps of:
forming a gold bump on the electrode pad of the first semiconductor chip,
forming a gold bump on the electrode pad of the second semiconductor chip,
forming an easily deformable gold stud bump on the gold bump of the first semiconductor chip or on the gold bump of the second semiconductor chip, and
pressing the semiconductor chips to each other while facing the first and second semiconductor chips with each other so that the electrode pad on the first semiconductor chip faces the electrode pad on the second semiconductor chip.
The semiconductor device manufactured according to the above method provides the same advantages as described according to the second aspect of the present invention.
The formation of the gold bumps can be achieved by a method of gold plating for example. The stud bumps can be formed by generally the same operation as the first bonding in the wire bonding step of the gold wire. Specifically, an end portion of the gold wire is drawn out of a tip portion of a capillary. The end of the gold wire is then heated into a ball of molten gold in a hydrogen flame or electric discharge. The capillary is then moved to above the gold bump of the first semiconductor chip, and lowered so that the ball of molten gold is pressed onto the gold bump. When the gold wire is bonded, while the gold wire is still soft, the capillary is raised to tear the gold wire off.
According to the preferred embodiment, the first semiconductor chip and the second semiconductor chip are pressed to each other via a resin adhesive placed between the first and second semiconductor chips.
The resin adhesive used in the above operation is an epoxy resin adhesive or a phenol resin adhesive for example. Each of them is a cold setting type adhesive, or a thermosetting type adhesive setting at a relatively low temperature such as about 100 degrees Celsius.
Other features and advantages of the present invention should become clearer from the detailed description to be made hereafter with reference to the attached drawings.